Because of their low cost, kraft and hemp papers have been widely used as backings for electrical-insulating pressure-sensitive adhesive tapes. Paper-like backings based on polymeric fibers, such as cold-drawn polyester fibers, have provided better electrical insulation, tear strength, and resistance to heat, but heretofore only at substantially increased cost. For example, U.S. Pat. No. 3,309,260 (Boese) discloses one such backing made from a randomly-intermingled mixture of cold-drawn and undrawn polyester fibers, the latter fusing under heat and pressure in order to knit the former into a coherent web. If the fiber-bonded web at this state were coated with a pressure-sensitive adhesive, it could not be wound on itself and unwound without delaminating and hence should first be unified with a resinous binder. Care must be taken in forming the web to avoid loose surface fibers which could produce an undesirable roughness after application of the resinous binder. To provide better dielectric and physical strength, a high-strength plastic film may be laminated to a paper or paper-like backing before applying a pressure-sensitive adhesive coating, but this increases the cost.
Another fibrous backing for pressure-sensitive adhesive tape is shown in U.S. Pat. No. 3,562,088 (Newman et al.). Referring to FIG. 1 of the drawing, a layer of acrylic resin 10 is cast onto a polypropylene film 12, and a carded web of polymeric fibers 14 such as nylon is laid into the acrylic resin layer. The three layers are hot-calendered between rolls 16 and 20 to provide a non-woven fibrous backing to which a pressure-sensitive adhesive layer 22 may be applied (FIG. 3). This tape should have excellent electrical-insulating properties, but at substantially increased cost as compared to ordinary paper-backed electrical-insulating tapes.